Thermal and kinetics studies of primary, secondary and tertiary alkanolammonium salts of 4-nitrobenzoic acid

Article abstract of DOI:10.1007/s10973-018-6975-7

Nitrobenzoic derivatives are chemically and biologically significant molecules, recently listed as active ingredients in the medical-pharmaceutic field. A series of p-nitrobenzoic acid salts were synthesized with different substituted alkanolamine (ethanolamine, diethanolamine and triethanolamine) via proton exchange reactions and characterized. Fourier transform infrared spectroscopy—FTIR-UATR, and a combination of thermal techniques (differential scanning calorimetry—DSC, and thermogravimetric analysis—TGA) with hot-stage microscopy were used in order to demonstrate the formation of salts and to analyse thermal stability and phase transitions. The aim of this study is to investigate thermal behaviour and kinetics of this class of compounds, previously poorly examined, which offers interesting phase transformations in the solid state. DSC indicated that the synthesized salts had very distinct melting points. Diethanolamine and triethanolamine used as cation in the formation of multicomponent systems with 4-nitrobenzoic acid lead to melting points near 100?°C, compared to compound based on ethanolamine. Calorimetric and thermogravimetric data indicate the absence of solvate forms in all studied compounds. TGA and kinetic experiments allowed the calculation of the activation energy, revealing that triethanolammonium salt has the highest stability in this studied series of compounds.

Full text of DOI:10.1007/s10973-018-6975-7

Journal of Thermal Analysis and Calorimetry
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Thermal and kinetics studies of primary, secondary and tertiary
alkanolammonium salts of 4-nitrobenzoic acid
Manuela Crisan1 Gabriela Vlase2
Elisabeta I. Szerb1 Titus Vlase2
•
•
•
Received: 6 September 2017 / Accepted: 5 January 2018
´
´
Ó Akademiai Kiado, Budapest, Hungary 2018
Abstract
Nitrobenzoic derivatives are chemically and biologically significant molecules, recently listed as active ingredients in the
medical-pharmaceutic field. A series of p-nitrobenzoic acid salts were synthesized with different substituted alkanolamine
(ethanolamine, diethanolamine and triethanolamine) via proton exchange reactions and characterized. Fourier transform
infrared spectroscopy—FTIR-UATR, and a combination of thermal techniques (differential scanning calorimetry—DSC,
and thermogravimetric analysis—TGA) with hot-stage microscopy were used in order to demonstrate the formation of salts
and to analyse thermal stability and phase transitions. The aim of this study is to investigate thermal behaviour and kinetics
of this class of compounds, previously poorly examined, which offers interesting phase transformations in the solid state.
DSC indicated that the synthesized salts had very distinct melting points. Diethanolamine and triethanolamine used as
cation in the formation of multicomponent systems with 4-nitrobenzoic acid lead to melting points near 100 °C, compared
to compound based on ethanolamine. Calorimetric and thermogravimetric data indicate the absence of solvate forms in all
studied compounds. TGA and kinetic experiments allowed the calculation of the activation energy, revealing that tri-
ethanolammonium salt has the highest stability in this studied series of compounds.
Keywords Alkanolammonium salts of 4-nitrobenzoic acid Á Thermal stability Á TG/DTG Á DSC Á Non-isothermal kinetics Á
Modified NPK method Á Protic alkanolammonium ionic liquids (APILs)
Introduction
tuberculous mycobacteria [10, 11]. Considering the present
and future prospects of nitro drugs based on the unique
properties of nitro group and its ability to form hydrogen
bonds, there is a compelling need for complex researches
in this controversial topic.
The interest in nitroaromatic compounds over the years as
important structural elements for the synthesis of industrial
chemicals, dyes, pigments or herbicides is well recognized
[1–4]. These compounds, despite the potential toxicity
issues [5, 6], have been known as therapeutics for
Parkinson’s disease, angina and insomnia [7–9], and
recently their use in infectious diseases has been devel-
oped. Among these compounds, 4-nitrobenzoic acid has
been listed as inhibitor agent for recognition of Mycobac-
terium tuberculosis complex and differentiation from non-
In pharmaceutical industry, the salts represent about a
half of all active pharmaceutical ingredients on the market,
bringing an improvement in drug’s physicochemical
properties (e.g. melting point, chemical and physical sta-
bility, solubility, bioavailability and dissolution rate). In
addition, selection of the appropriate anion/cation combi-
nation, keeping anion constant and changing the counterion
or introduction of specific functional groups on the cation
or anion can affect the physicochemical and biological
properties of salt. An increasing number of 4-nitrobenzoate
multicomponent complexes formed with organic cations
such as aliphatic amines [12], guanidine [13, 14], 8-hy-
droxyquinoline [15], amino acids [16, 17], piperazine [18],
pyridine [19], imidazole [20] are developed and studied
& Gabriela Vlase
gabriela.vlase@e-uvt.ro
1
Institute of Chemistry Timisoara of Romanian Academy, 24
Mihai Viteazul Boulevard, Timisoara 300223, Romania
2
Research Center: Thermal Analysis in Environmental
Problems, West University of Timisoara, Pestalozzi Street
16, Timisoara 300115, Romania
123

M. Crisan et al.
recently in the literature as promising NLO materials or
active pharmaceutical ingredients.
FTIR spectra were performed using PerkinElmer Spec-
trum 100 FTIR spectrometer with UATR, in the range
4000–650 cm-1 (see Fig. 1). Single-crystal X-ray diffrac-
tion analyses for all studied compounds were done at room
temperature on a Xcalibur ‘‘Oxford Diffraction’’ diffrac-
tometer equipped with CCD area detector and a graphite
monochromator utilizing MoKa radiation. Thermal beha-
viour was determined using a Diamond TG/DTA Perk-
inElmer in dynamic air atmosphere (synthetic air 5.0 Linde
Gas with flow 100 mL min-1). The experiments were
carried out from 25 °C up to 500 °C, at heating rates
b = 5, 7, 10, 12 and 15 °C min-1, using open Al crucible
(see Fig. 2). The phase transitions of the microcrystalline
solids were observed using an Olympus BX53M polarizing
microscope (POM) equipped with Linkam hot stage. Ima-
ges of the various crystalline phases were recorded with an
Olympus UC90 camera, using a filter with k = 530 nm.
Differential scanning calorimetry study was performed
with one PerkinElmer Diamond DSC using aluminium
closed pans with heating and cooling rate of 10 °C min-1
in nitrogen 20 mL min-1 (5.0 Linde Gas).
In the last years, searching for new compounds with
potent biological activity and low toxicity seems to be a
new trend in biosciences. Our previous studies highlighted
the alkanolamines as promising molecules used as salt-
forming agents and revealed the structural complexity and
low toxicity of the system represented by alkanolamine
salts with different aromatic carboxylic acids. These salts
have provided interesting supramolecular networks with
different topologies according to structural variation of
counterions [21, 22], lower toxicity comparatively with
aliphatic amines and heterocyclic amine salts [23] and
biological activity in plants [24–26].
In this work we have focused on investigation of thermal
behaviour and kinetics by thermal analysis techniques
(DSC and TG/DTA) of alkanolammonium salts of 4-ni-
trobenzoate in order to highlight the nature of their solid
state. Both counterions have active pharmaceutical prop-
erties and are economically, commercially available, which
makes them promising for the design of new dual active
compounds.
Results and discussion
Materials and methods
Synthesis and physicochemical characterization
All reagents, 4-nitrobenzoic acid—4-NBA, and the bases
(ethanolamine—MEA,
diethanolamine—DEA,
tri-
A series of alkanolammonium salts of 4-nitrobenzoic acid
were prepared according to ‘‘rule of three’’, via proton
exchange reaction, each base having a sufficiently high pKa
to react with 4-nitrobenzoic acid (Table 1). In this princi-
ple, the degree of ionization is considered to be a critical
parameter, the DpKa (pKa of base - pKa of acid) value
being important for predicting salt or co-crystal formation.
DpKa value greater than 3 leads to salt formation, while
DpKa less than 0 lead to co-crystal formation [27].
The reaction has occurred rapidly, with high yields
(90–92%). The compounds are water soluble and with well-
defined melting points not exceeding ca. 0.5 °C, which is an
indicative of their high purity. All synthesized alkanolamine
salts have showed lower melting points compared with
4-NBA. The values of purity determined by spectrophoto-
metric method were between 99.12 and 99.36% (Table 2).
UV spectroscopic data showed similar kmax values for salt
and acid (4-NBA kmax = 273.6 nm) that confirm the exis-
tence of the same anion in both compounds.
ethanolamine—TEA) were purchased in analytical purity
grade from Fluka AG (Buchs SG) and used as received.
The salts were prepared in a 1:1 molar ratio, dissolving
the acid and the appropriate alkanolamine in acetone and
then mixing the two solutions. The reaction was completed
after 2 h of continuous stirring at room temperature, the
salts precipitating in a crystalline state according to their
solubility in acetone. The compounds were collected by
filtration, washed with acetone and then dried in a vacuum
for 3 h. The salts were recrystallized from acetone or
ethanol. The suitable crystals for single-crystal X-ray
diffraction analysis were obtained by slow evaporation at
room temperature.
The purity of all studied salts was established via an UV
spectrophotometric method, which consisted in determin-
ing the specific extinction (absorbance at kmax of a solution
1 gl-1 in a cell of 1 cm width) of the free acid (ea) and the
corresponding salt (es), by measuring the UV absorbance in
NaOH 0.1 M at kmax
.
The evidence of proton transfer and molecular interac-
tions between alkanolammonium cation and 4-nitroben-
zoate anion is illustrated in the FTIR-UATR spectra
(Fig. 1). The formation of salts was identified by: (a) the
presence of asymmetric and symmetric m(COO-) vibrations
occurring at 1650–1540 and 1450–1360 cm-1, (b) N–H
vibrations attributable to –NH?3 , –NH2? and –NH?,
es Á Ms
Purity ¼
Á 100
ð1Þ
ea Á Ma
where Ms and Ma are the molecular weights for the salt and
free acid, respectively.
123

Thermal and kinetics studies of primary, secondary and tertiary alkanolammonium salts of…
A
B
C
4000.0
3600
3200
2800
2400
2000
1800
1600
1400
1200
1000
800 650.0
Wavenumber/cm-1
Fig. 1 FTIR-UATR of 4-nitrobenzoic acid salts prepared with
ethanolamine (A), diethanolamine (B) and triethanolamine (C). A—
C9H12N2O5 (228.20); FTIR-UATR (1950mmax/cm-1): 2930, 2689
(–NH?3 ), 1646, 1612, 1556, 1385 (–COO-), 1102, 1068, 1021, 1009
(C–OH); B—C11H16N2O6 (272.26); FTIR-UATR (1942 mmax/cm-1):
3031, 2738 (–NH?2 ), 1615, 1567, 1377 (–COO-), 1098, 1066, 1035,
1007 (C–OH); C—C13H20N2O7 (316.31); FTIR-UATR (1980 mmax
/
cm-1): 2722 (-NH?), 1615, 1560, 1548, 1376 (–COO-), 1095, 1071,
1037, 1008 (C–OH)
8
105
100
(a) 5
(b)
105
100
10
9
1
TG
7
6
DTG
TG
0
90
80
70
60
50
DTG
8
90
80
70
60
50
40
–5
10
5
4
3
2
1
–5
7
6
–10
–15
5
4
3
15
40
30
20
10
0
0
2
–20
–20
–25
–30
–1
–2
–3
–4
1
0
Normalized heat flow
–25
–30
30
20
Normalized heat flow
–1
–2
35
100
150
200
250
300
350
400
450
505
300
200
250
400
450
510
350
100
150
35
Temperature/°C
Temperature/°C
1
10
(c)
105
100
9
8
TG
DTG
90
–5
10
15
7
80
70
6
5
4
60
50
40
3
2
–20
–25
1
0
–1
–2
–3
30
20
Normalized heat flow
–30
35
100
150
200
250
300
350
400
450
505
Temperature/°C
Fig. 2 TG/DTG/feat flow (in air) of 4-nitrobenzoic acid salts prepared with ethanolamine (a), diethanolamine (b) and triethanolamine (c) (in air
at heating rates b = 10 °C min-1
)
stretching of solid ammonium salts mNH3?= 3200–2700 cm-1
NH2?= 3100–2700 cm-1, mNH?= 2800–2400 cm-1 and
,
(c) the absence of stretching band mC=O = 1710–
m
1680 cm-1, characteristic for the carboxyl group [29, 30].
123

M. Crisan et al.
Table 1 Tabulated DpKa of
alkanolammonium salt/protic
ionic liquid of 4-nitrobenzoic
acid
No.
A
Compound name
Abbreviation
4-NBA MEA
Structure
DpKaa
Monoethanolamine
4-nitrobenzoate
6.06
COO
+
H3N-CH2-CH2-OH
NO2
B
Diethanolamine
4-nitrobenzoate
4-NBA DEA
5.52
COO
+
H2N(CH2-CH2-OH)2
NO2
COO
C
Triethanolamine
4-nitrobenzoate
4-NBA TEA
4.32
+
HN(CH2-CH2-OH)3
NO2
aDpKa = pKa (base) - pKa (acid) was calculated using pKa data from PubChem Compound Database
[28]. All pKa values have been determined in aqueous solutions
Table 2 The purity values of studied compounds
Sample
k
max/nm
Average specific extinction acid
ea
Average specific extinction salt
es
Purity, p/%
4-NBA MEA
4-NBA DEA
4-NBA TEA
273.6
273.4
273.4
58.42
42.45
35.59
30.67
99.22
99.24
99.36
The absorption bands reflected in FTIR-UATR spectra
confirmed the conversion of carboxyl group of 4-NBA and
amino group of alkanolamine into carboxylate and
ammonium groups, indicating the complete proton transfer
in studied compounds.
melting point compared to compounds B and C. Therefore,
the force of attraction between the molecules affects the
melting point of a compound.
Thermogravimetric analysis of compounds A–C
The crystallographic data of A–C compounds have
revealed that the salt unit serves as building block for
supramolecular architecture. The anions and cations are
self-assembled via ionic N–HÁÁÁO and normal O–HÁÁÁO
hydrogen bonds, compound A showing stronger inter-
molecular interactions [21], which leads to a higher
The TG/DTG/DTA curves of studied compounds recorded
in air at a heating rate of 10 °C min-1, and thermoana-
lytical data are presented in Fig. 2 and Tables 3 and 4,
respectively. The thermoanalytical data obtained at 5
heating rates, b = 5, 7, 10, 12 and 15 °C min-1 were used
123

Thermal and kinetics studies of primary, secondary and tertiary alkanolammonium salts of…
Table 3 TG data of compounds A–C
b Á da=dT ¼ kðTÞ Á fðaÞ
ð3Þ
Compound
A
Process
Tonset/°C
Tend/°C
Dm/%
Equation (3) is the starting point for all the data pro-
cessing methods used in kinetic analysis. The TG/DTG
curves furnish directly the da/dT, a and T for different
conversion degrees determined.
a
b
a
a
189.7
229.75
202.2
206.5
229.75
274.9
288.7
291.9
55.4
39.24
50.6
54.1
B
C
We used four different kinetic methods:
•
•
Differential isoconversional by Friedman [35] (FR)
lnðb Á da=dTÞa¼ ln½A Á fðaފaÀE=RT
ð4Þ
Integral isoconversional by Flynn–Wall [36] and
Ozawa [37] (FWO)
Table 4 Heat flow data of compounds A–C
Compound Process Tonset
°C
/
Tend
°C
/
Tpeak
DH/J g-1
ln b ¼ ln A=½R Á gðaފ À 5:331 À 1:052 Á E=R Á T
ð5Þ
a
A
B
C
a
b
c
a
b
c
a
b
c
165.0
176.1
234.3
282.9
87.4
171.4
216.2
270.7
82.6
166.1 4.3
66.7 2.2
R
where g(a) = [da/f(a)] is the integral form of the
194.9
251.7
79.5
0
85.1 3.8
conversion function.
5.2 0.9
•
The Kissinger–Akahira–Sunose [38, 39] method (KAS)
103.3
225.1
88.5
114.1
302.4
94.5
106.2
277.2
90.83
103.0
272.7
97.8 4.3
with Eq (6):
- 825.1 5.7
10.7 0.7
À
By these three methods, the plotting of the left member
Á
ln b=T2 ¼ ln½A Á R=E Á gðaފ ÀE=R Á T
ð6Þ
97.4
111.2
302.1
91.2 4.6
217.7
- 849.9 18.8
of Eqs. (4), (5) and (6) versus 1/T, the value of the acti-
vation energy will be obtained from the line slope. The f(a)
remain implicit, so these methods are ‘‘model free’’.
By plotting the values of the activation energy E versus
the conversion degree, the diagrams in Fig. 3 were
obtained. Variations of E versus a exceed 10%, and these
variations are non-monotonous. Therefore, a mean value of
the activation energy is reasonable only for comparison
(see Table 6).
for providing the kinetic parameters of the degradation
process (FR, FWO and modified NPK kinetic methods).
In the thermogravimetric study, similar behaviour for all
samples was found. Samples B and C show one highly
exothermic decomposition stage in the range of
150–300 °C.
Kinetic study
•
The modified NPK method [40–48]
The kinetic analysis was performed using the TG data in air
for the substance’s decomposition at five heating rates: 5,
7, 10, 12 and 15 °C min-1. To perform the kinetic analysis
of the TG experimental data, three isoconversional meth-
ods were used, a differential one and two integral ones,
respectively; fourth method elaborated by Sempere et al.
[31, 32] and modified and developed by Vlase et al.
[33, 34], the nonparametric kinetics method (NPK), in
order to obtain kinetic parameters.
According to Eq. (3), the experimental data are plotted
in a 3D coordinate system (da/dT, T, a) and then interpo-
lated for obtaining the reaction rate surface. This surface is
partitioned as a i 9 j matrix M.
È
É
È
À ÁÉ
:
M ¼ mij ¼ fðTiÞ g aj
ð7Þ
The NPK method uses the singular value decomposition
(SVD) algorithm to decompose matrix M into two vectors
[49]. The matrix M is decomposed as follows:
It is generally accepted that the reaction rate can be
expressed as a product of two separable functions, k(T)
dependent only on the reaction temperature T and f(a)
dependent on the reaction degree a, i.e.:
M ¼ Uðdiag sÞVT
and the first columns of U and V matrix, respectively, u1
and v1 are analysed for determining the kinetic model, i.e.:
ð8Þ
da=dt ¼ kðTÞ Á fðaÞ
ð2Þ
u1 ¼ gðaÞ
and a temperature dependence, i.e.:
ð9Þ
where t is the reaction time. Under non-isothermal condi-
tion usually the reaction temperature depends linearly on
time, T = Ti ? bt, where b is the constant heating rate, so
Eq. (1) becomes:
v1 ¼ fðTÞ
ð10Þ
123

M. Crisan et al.
(a)
(b)
(c)
3 × 105
3 × 105
3 × 105
2 × 105
1 × 105
0
2 × 105
1 × 105
0
2 × 105
1 × 105
0
0
0.2
0.4
0.6
0.8
0
0.2
0.4
0.6
0.8
0
0.2
0.4
0.6
0.8
degree of conversion
Activation energy (FR) vs. degree of conversion
degree of conversion
Activation energy (FR) vs. degree of conversion
degree of conversion
Activation energy (FR) vs. degree of conversion
(a)
3 × 105
2 × 105
1 × 105
(b)
(c)
4 × 105
3 × 105
3 × 105
2 × 105
2 × 105
1 × 105
1 × 105
0
0
0
0
0.2
0.4
0.6
0.8
0
0.2
0.4
0.6
0.8
0
0.2
0.4
0.6
0.8
degree of conversion
Activation energy (FWO) vs. degree of conversion
degree of conversion
degree of conversion
Activation energy (FWO) vs. degree of conversion
Activation energy (FWO) with error barr
(a)
(b)
(c)
3 × 105
3 × 105
3 × 105
2 × 105
2 × 105
2 × 105
1 × 105
1 × 105
1 × 105
0
0
0.2
0.4
0.6
0.8
0
0
0
degree of conversion
Activation energy (KAS) vs. degree of conversion
0
0.2
0.4
0.6
0.8
0.2
0.4
0.6
0.8
degree of conversion
Activation energy (KAS) vs. degree of conversion
degree of conversion
Activation energy (KAS) vs. degree of conversion
Fig. 3 Activation energy with errors bar versus conversion degree, determined by three different isoconversional methods for compounds A–C
Table 5 Mean activation
energy according to modified
NPK method
Compound
A
Process
Main
k/%
A/s-1
E/kJ mol-1
n
m
Corr.
E /kJ mol-1
78.7
18.8
3.35 9 1010
1.47 9 109
92.1 15.1
86.6 5.1
1
–
–
1
0.996
0.996
91.6 20.5
Secondary
a
Secondary
b
2.4
5.69 9 1013
113.8 0.2
1
1
0.997
B
C
Main
75.2
24.5
4.47 9 109
1.03 9 109
73.1 11.1
73.1 6.0
1
–
–
1
0.994
0.993
73.1 17.1
Secondary
a
Main
86.1
11.8
1.31 9 1012
2.66 9 1010
112.4 19.8
106.4 6.2
1
–
–
1
0.998
1
111.3 26.8
Secondary
a
2.2
1.19 9 1010
93.3 0.8
1
1
0.999
For the temperature dependence, the Arrhenius equation
is selected, whereas for the kinetic model, we suggest the
´
use of Sestak–Berggren equation [50]:
where am describes the influence of physical phenomenon
related with the presence of reaction product and (1 - a)n
describes the chemical phenomenon which involves the
remainder reactant (1 - a).
n
gðaÞ ¼ amð1 À aÞ
ð11Þ
123

Thermal and kinetics studies of primary, secondary and tertiary alkanolammonium salts of…
Table 6 Mean activation
Sample
Method
FR
energy (E /kJ mol-1
determined by the four different
kinetic analysis methods
)
FWO
KAS
94.0 6.7
NPK
91.6 20.5
A
B
C
91.4 5.1
90.7 15.0
114.4 12.9
97.3 6.9
77.5 7.3
72.9 7.6
73.1 17.2
120.2 12.6
111.4 13.1
113.0 27.2
5.904
(b)
20
(a)
5
4
15
3
2
1
0
10
5
–1
–2
–3
–4
0
–5
–10
–5
–6.246
–15
25
40
60
80
100
120
140
160
180 190
25 30
40
50
60
70
80
90
100
110 120
130
140 150 155
Temperature/°C
Temperature/°C
4
2
(c)
0
–2
–4
–5
25
40
50
60
70
80
90
100
110
120
130
140
Temperature/°C
Fig. 4 DSC curves showing different situations at heating solid line, respectively, at cooling dashed line for A–C alkanolammonium salts
If the decomposition process is a result of two or more
Calorimetric analysis of compounds A–C
simultaneous steps, it means that r = R ri and conse-
quently the matrix M becomes:
DSC curves and melting characteristics of A–C compounds
under N2 atmosphere are illustrated in Fig. 4 and Table 7.
DSC curves of compounds A are characterized by the
presence of one endotherm peak associated with melting
with decomposition and the absence of solvate forms
(Fig. 4), while for compounds B and C, a phase transition
crystal–crystal was observed before the endotherm peak
associated with melting. The transition between different
phases was examined under Olympus BX53M polarizing
microscope (POM) equipped with Linkam hot stage
(Fig. 5).
M ¼ RMi
ð12Þ
The contribution of each step to the observed process is
expressed by its variance, k, so that Rki = 100%. By the
data processing with the NPK method, kinetic parameters
were obtained (Table 5).
Activation energy value, presented in Table 6, indicates
a reasonable agreement between the E values obtained by
four different kinetic methods. The advantages of the NPK
methods are: no supplementary approximations necessary
for solving the temperature, and the respective conversion
integrals and the kinetic description were complete.
123

M. Crisan et al.
Table 7 DSC characteristics of compounds A–C
Compound
Heating
Process
Cooling
Process
Tonset/°C
Tend/°C
Tpeak
DH/J g-1
Tonset/°C
Tend/°C
Tpeak
DH/J g-1
A
B
a
a
b
c
a
b
172.44
54.83
76.15
102.80
88.23
97.64
176.77
65.14
175.02
61.66
- 207.03
44.66
a
a
135.77
67.95
134.49
52.23
135.51
64.66
134.24
84.78
78.07
77.18
- 100.20
- 120.45
- 44.44
- 140.00
107.23
92.33
105.54
90.95
C
a
61.79
44.19
56.15
29.82
105.62
102.17
(a)
25 ºC
87 ºC
88 ºC
104 ºC
108 ºC
62 ºC cooling
(b)
86 ºC
87 ºC
92 ºC
102 ºC
110 ºC 65 ºC cooling
Fig. 5 Optical micrographs of the crystalline solids B a and C b showing the transitions between different phases taken with filter k = 530 nm
determined using four different methods, the values
Conclusions
obtained by KAS and NPK being in good agreement.
Kinetic results showed differences in activation energy
between the studied compounds which increased in the
order of C [ A [ B.
New alkanolammonium salts based on pharmaceutical acid
4-NBA have been synthesised, and their thermal behaviour
were compared. The studied compounds display various
phase transitions in the solid state, which have been
examined by thermal analysis, differential scanning
calorimetry and hot-stage microscopy. Since in this study
the 4-nitrobenzoate anion is a constant, differences of the
solid state can be attributed to the alkyl and hydroxyl
groups of the cation. When the cation possesses only one
hydroxyl group (4-NBA MEA), the salt shows one endo-
therm peak associated with melting and decomposition.
Substitution of H atoms linked to N atom in alkanolamines,
with hydroxyl and ethyl groups, leads to different hydro-
gen-bonding network in final compounds and exhibits a
phase transition crystal–crystal below the melting point.
Compounds B and C show low melting points near 100 °C,
compared to compound A which melts at 175 °C. The
activation energies of the decomposition reactions were
Acknowledgements Authors thank the bilateral Moldova-Romanian
project 16.80013.5007.04/Ro and the Romanian National Authority
for Scientific Research and Innovation, CCCDI-UEFISCDI, project
PN3-P3-217/24 BM/19.09.2016
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